<style>
    button{
        margin: 0 auto;
        width: 100px;
    }
</style>

<form id="add-form" class="form-horizontal" role="form" data-toggle="validator" method="POST" action="">
    <button type="button" id="plupload-avatar" class="btn btn-danger btn-lg plupload" data-input-id="c-avatar" data-mimetype="image/gif,image/jpeg,image/png,image/jpg,image/bmp" data-multiple="false" data-preview-id="p-avatar"><i class="fa fa-upload"></i> 上传</button>
</form>